Regulating device for centrifugal compressors



March 1, 1938.. p,-A BANCEL ET AL 2,109,964

REGULATING DEVICE FOR CENTRIFUGAL COMPRESSORS Filed June 5, 1955 I 1 ll 1 f5 INVENTORS 1 I PauZZLBalweZ fill fin am HOf man.

11 THEIR ATTORNEY.

REGULATMIG DEVHCIE FUR CIEN'EG CUMUPRESSOR Paul A. Bancel, Montclair, N. 3., and Paul Hodman, Easton, Pen, asslgnors to lngcrsoll-lltand Company, .lersey City, N. l, a corporation of New Jersey Application June 5, 1935, Serial No. 25,006 2 claims. '(ci. 6Z1l5) Our invention relates to regulating means, and particularly to regulating means for centrifugal compressors that are intended to carry varying effective loads at high .but virtually constant speed.

An object of the invention is to accomplish regulation of a centrifugal compressor under these conditions, so that it can 'be'operated at reduced load in a system without becoming unstable as the load decreases to what would otherwise be a critical point; This tendency is a characteristic of the centrifugal compressor, which runs well at full load or somewhat above and below this, but shows a derangement of function when the load is materially diminished.

In practice the invention is intended for water vapor refrigerating systems, which comprise a vessel or evaporatoig'to which .water is admitted to be partly vaporized and thus chill themain body of the liquid. The water vapor formed in said vessel must be evacuated and is forced into a condenser to be liquefied. The use of a centrifugal compressor at high speed for this purpose has: decided advantages, except that the aforesaid .tendency to become unstable results in pumping or surging when the system is operated at-part load; that is, the volume delivered at lighter loads and the suction and compression obviated, and is apt to appear when the load bepressure will fluctuate to a marked extent. This surging of the compressor under rated chilled water conditions is accompanied by corresponding surges in power with apossible harmful effect on the driving gears for the compressor and the motor.

However,.with water vapor refrigeration, this tendency of the compressor to develop surging at light loads is in a measure counteracted and neutralized over a large 'partof the load range by changing conditions in the evaporator. In other words, as the load falls and the compressor continues to revolve without change 'in speed to evacuate water vapor from the evaporator, there is, with proper regulation of'the vmedium for cooling the condenser, a drop in the pressure of the water vapor in the evaporator accompanied by a. corresponding decrease in the density of the vapor. This lower density is attended. by an increase of specific volume, and the consequence is that the surging point does not occur where it ordinarily would, but is displaced somewhat towards a smaller load value. But the objectionable tendency remains because it is not wholly comes very light.

The principal object of this invention is to 1 are expressed.

entirely obviate surging of the fluid being compressed at light loads resulting from the use of a centrifugal compressor in connection with water vapor refrigerating. apparatus, and to obtain smoothness and certainty of operation and 5 thus eliminate all undue wear and risk of damage to the blower and its driving connections.

A further object is to prevent surging of the vapor in' the compressor by a simple, inexpensive and easily controlled by-pass from the condenser 10 to the inlet side of the compressor, without any special controlling means for the motor, or a damper for the compressor, or other special appliances for altering the usual mode in which the compressor operates, and without changing 15 the nature of its connection to the evaporator or condenser. By means of this connection the immediate load on the compressor can be adjusted so as to be higher than refrigeration output conditions require, while the eflective load, 20 which consists in the amount of water vapor permanently removed from the evaporator and actually liquefied in the condenser, is much less.

The nature and advantages of the invention are fully disclosed in the ensuing specification, 5 but changesin the construction and mode of operation disclosed may be adopted to the full extent consistent with the scope and general meaning of the terms in which the appended claims 30 On the drawing Figure 1 is an outline of apparatus according to this invention, and

Figure 2 shows a modification.

The numeral i indicates an evaporator to which water is admitted by way of a spray nozzle 35 2 therein, connected to a supply pipe 3. y In operation the pressure'in this evaporator will be low enough to cause some of the water to be converted into vapor as it enters and the heat of vaporization which isthen rendered latent rep- 40 resents an amount of heat that is taken from exhausts the water vapor from the evaporator,

being revolved at constant high speed by an electric .motor or a steam turbine indicated at 6. 5o

' The water vapor enters-the casing ofthe compressor by way of a suction passage 1 and is discharged through an outletii into a suitable condenser 9. Step up gearing l0 may be arranged in the line of the shaft ll connecting the motor 55 and compressor to revolve the latter at the speed required.

With this construction excellent results can be obtained, the water being cooled to the extent .desired and passing out of the evaporator from full load down to part load through a considerable range of operation. This function makes the compressor self-regulating in the system because, as the load drops and the final temperature of the chilled water also drops, the compressor takes less power, although the speed remains fixed. On a decrease in load, the first result is that the pressure in the evaporator falls to some extent and then the density of the water vapor decreases while its specific volume increases. At higher load the Water vapor is compressed at greater density and lower specific volume, while at light load it is compressed at less density and greater specific volume. The machine will therefore operate over full load range and down to a given part load more as a constant volume blo'wer. Hence; the surging point is displaced from a large part of the working range of the blower and is not apt to be reached at all except at extremely light loads. This valuable characteristic of the compressor in a water refrigerating system is fully set forth in the application of Paul A. Bancel for patent on Method and apparatus for refrigerating purposes, Serial No. 22,003, filed May 17,1935.

Surging, as stated above, renders the load on the compressor unstable when it occurs and the volume delivered and the pressures in the evaporator, compressor and condenser fluctuate very much. The operation of the compressor thus becomes uneven and fluctuations in the refrigerating effect produced in the evaporator and in the power to drive the motor 6 also take place. This tendency of the compressor load to surge or to become unstable at the smaller load levels can be eliminated entirely by introducing low pressure vapor, preferably from the compressor discharge or from the condenser, into the evaporatorand compressor in order to maintain the pressures in the evaporatorand compressor above critical minimum values. In the'construction illustrated in Figure 1 the evaporator l is connected at a point spaced away from the port I through a pipe l2 to the condenser 9. This bypass pipe may have a casing l3 with a valve therein so that it can be shut when full load operation is desired, the valve having an outside knob H so that it can be manipulated. Therefore, during full load and over a considerable range of part load, so long as self-regulation of the apparatus can be depended upon, the bypass i2 is kept closed, but when a much diminished load that is light enough to make surging likely is reached, this by-pass is opened. The result is that some water vapor is withdrawn from the condenser, before it is liquefied and conducted back into the evaporator and, along with freshly formed vapor, re-enters the compressor. The reintroduced vapor is preferably drawn from within the condenser for at that point it is somewhat cooler than at the compressor discharge 8. Any condensate in the returned vapor falls into the water at the bottom,

75 and thus does not reach the compressor. The

reintroduced vapor is, however, obviously warmer and at higher pressure than when it was drawn out of the evaporator i. The return vapor thus introduces heat into the evaporator and increases the volume, pressure and temperature of the vapor formed therein and also increases the immediate load on the blower, although the load on the system may grow even smaller, because a greatly reduced amount of water vapor is permane'ntly exhausted from the evaporator .4. The blower or centrifugal evaporator thus performs an amount of work that may be equal to that performed at some greater part load above the possible surg ng point. Nevertheless the system is being operated at a smaller load, for only part of the water vapor forced into the condenser remains there to be liquefied.

In other words, regulation at light load on the system is achieved by increasing the intake and output of the blower, thus making the work performed by the blower itself relatively heavy, but the net or effectiveload on the system is merely so much of the vapor formed in the evaporator and extracted by the compressor as is actually liquefied in the condenser 9.

Any tendency remaining of the centrifugal compressor to become unstable at light loads in a water refrigerating system is thus entirely overcome. The regulating connection is quite effective and certain in its effects, costs very little to install and does not interfere with the opera- .tion-of the system when the full refrigerating effect is, needed.

The connection H2 in Figure 1 runs from the condenser to the evaporator. In Figure 2 it communicates with the compressor beyond the inlet and the first stage thereof. This arrangement increases the load on the remaining stages to a sufficient extent, so that surging does not take place. No load of'returned vapor need be added to the first stage, because the pumping point thereof is somewhat lower than for the other stages. In both cases the pipe l2 must be joined pressure against which the compressor dis-- charges will decrease for, now, less vapor is actually condensed in the condenser. Thus the compression ratio of the compressor may be decreased and prevented from reaching the critical maximum at which surging of the load occurs. The compressor in the apparatus shown in the drawing will act to regulate itself until light loads are reached, as set forth in the application of Paul A. Bancel above cited. But with the improvement hereinbefore described, all tendency is obviated for the compressor to surge belowthe range where such self-regulation is accomplished, and the refrigerating system can be fully controlled from full load down to the point where very little refrigeration is. performed by it.

Of course the valve in the casing I3 may be connected to means for opening and closing it automatically when the by-pass I2 is to be put into or out of service.

Another advantage of the by-pass I2 is that will prevent the water in the evaporator from being cooled too much, and help to prevent freez- 11 The. vapors liquefied in the condenser may be drained back into the evaporator, or the vaporization loss of the water in the evaporator can aioaeoe be made up in any other well known manner.

We claim: I

1. In refrigerating apparatus, the combination with an evaporator, a plural stage compressor to remove vapor from the evaporator, a condenser to which the compressor discharges, and motor means adapted to drive the compressor at constant speed, of means to introduce vapor discharged by the compressor into thecompressor between stages ther of to increase the pressure in the compressor ans to decrease the pressure in the condenser again h which the compressor discharges thereby to pr.=-:ent surging of the load in the compressor.

2. In refrigerating apparatus, the combination with an evamrator, a plural stage compressor to remove vapor from the evaporator, a condenser to which the compressor discharges, and motor means adapted to drive the compressor at constant speed, of a by-pass conduit leading from the condenser and opening into the compressor between stages thereof, and a valveto open the conduit to introduce vapor from the condenser into the compressor for increasing the pressure a in the compressor and for decreasing the pressure in the condenser thereby to stabilize the load in the compressor.

PAUL A. BANCEL. PAUL HOFFMAN. 

